At the hippocampal Schaffer collateral to CA1 synapse long term enhancement of synaptic transmission may underlie memory storage. Synaptic strengthening can become dysfunctional in Alzheimer's disease, Parkinson's Disease and other neurodegenerative disorders as synapses decline or are retracted. One important aspect of synaptic plasticity after frequent activity is the enhanced insertion of AMPA receptors into the postsynaptic plasma membrane. In addition to postsynaptic changes, however, long lasting presynaptic changes occur. During development vesicle pool sizes increase to handle more frequently occurring synaptic events, but the role of long lasting presynaptic plasticity at the adult CA1 synapse after high frequency events has been incompletely studied. One form of presynaptic plasticity is a long lasting change in the rate at which vesicles recover to readily releasable pools. During synaptic activity, a decrease in availability of vesicles for release manifests as synaptic depression; the rate at which vesicle pools recover after synaptic depression affects the response capabilities of the synapse and is therefore amenable to plasticity. We have found previously that the anti-death protein Bcl-xL acts not only to prevent somatic death in neurons, but to prevent synaptic depression acutely and over the long term. In part Bcl-xL functions to enhance the rate of recovery of the readily releasable pool of synaptic vesicles. Our recent preliminary data show that Bcl-xL binds directly to vesicle membranes, enhancing the rate of vesicle re-uptake from the plasma membrane during synaptic activity. In addition to vesicle re-uptake, however, our data also show that Bcl-xL, by its regulation of synaptic metabolism, may affect mobilization of vesicles from the reserve pool. The hypothesis of this application is that Bcl-xL causes long term changes in synaptic strength by enhancing mitochondrial efficiency to increase mobilization of a reserve pool of synaptic vesicles. In this proposal, we will study acute and long term changes in mitochondrial efficiency by measuring ongoing changes in ATP levels and oxygen uptake in living presynaptic boutons undergoing stimulation in hippocampal neurons both in culture and in vivo. Using imaging and recording techniques, we will differentiate the effects of Bcl-xL on vesicle re-uptake at the plasma membrane from those on reserve pool mobilization to determine the effect of mitochondrial efficiency on different vesicle re-accumulation strategies. We will also study the effects of Bcl-x on long term changes in vesicle pool depression brought on by inhibition of neuronal excitability. In contrast to decreased activity, increased synaptic activity may be associated with enhanced ATP availability, directly promoting phosphorylation and activation of downstream targets such as synapsin that rapidly release vesicles from the reserve to the readily releasable pool. Our studies will shed light on basic properties that strengthen the synapse during learning and memory or weaken it during synaptic dysfunction or neurodegeneration.